The present invention relates to a sustained release preparation which permits gradual release of volatile ingredients such as pheromones, repellents, perfumes and insecticides and which is effective, in particular, as a sustained release dispenser for releasing pheromones to control occurrence of harmful insects.
There has been required for the development of a pharmaceutical or agricultural dispenser which permits gradual release of a volatile ingredient to ensure a long lasting effect thereof. The mating disruption method must satisfy this requirement, this method is utilized for control of agriculturally harmful insects through release of, for instance, a sex pheromone at a predetermined concentration over a long time period. The term "mating disruption method" herein means a method comprising, 1) making a sex pheromone drift in fields at a concentration substantially higher than that released from harmful insects, 2) to thus lower the communication ability of the harmful insects such as an ability of male or female insects to recognize the individual opposite sex and to confirm the positions thereof, 3) and to thereby disturb mating of the insects. There has been used a sustained release dispenser containing a pheromone as a physiologically active substance of a harmful insect as a communicationdisturbing agent. Such sustained release dispenser may be in various forms, for instance, those packed in microcapsules as disclosed in U.S. Pat. Nos. 2,800,457, 2,800,458 and 3,577,515; those supported by carriers as disclosed in U.S. Pat. No. 4,160,335; those packed in tubes as disclosed in U.S. Pat. Nos. 4,600,146 and 4,834,745; or those packed in bottle-like containers as disclosed in European Patent No. 273,197. The sustained release dispenser packed in a container having a large capacity, i.e., a reservoir type one is one of the leading mainstream thereof.
Containers for such reservoir type sustained release dispensers have been produced from a variety of plastics selected depending on the physical properties of individual volatile ingredients. When using such a reservoir type dispenser, a liquid and volatile ingredient absorbed in the plastic wall of the container can be vaporized from the outer surface thereof and diffuse into the air. Therefore, the release rate thereof is approximately proportional to the outer surface area of the reservoir which absorbs the liquid ingredient. The release rate has conventionally been controlled by adjusting the size of the outer surface area through appropriate selection of plastic materials for the reservoir and shapes thereof. However, the reservoir type sustained release dispenser suffers from a problem in that the amount of the liquid remaining in the reservoir decreases as the ingredient is released and this results in a decrease of the surface area wetted with the liquid ingredient through absorption thereof and hence the reduction in the amount of the drug released therefrom during the latter half of the ingredient-release, as clearly described in J. Economical Entomology, 78, No. 6, 1985.